Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:

  • The length of num is less than 10002 and will be ≥ k.
  • The given num does not contain any leading zero.

Example 1:

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Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

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Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

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Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

Solution:

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class Solution {
    public String removeKdigits(String num, int k) {

        int size = num.length();

        if (k == size) {
            return "0";
        }

        Stack<Character> stack = new Stack<>();

        int counter = 0;

        // 1432219 // 1219
        // 439
        // stack [1,3]
        // counter = 2
        // k = 2
        // size = 7
        while (counter < size) {
            while (k > 0 && !stack.isEmpty() && stack.peek() > num.charAt(counter)) {
                stack.pop();
                k--;
            }
            stack.push(num.charAt(counter));
            counter++;
        }

        while (k > 0) {
            stack.pop();
            k--;
        }

        StringBuilder sb = new StringBuilder();

        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }

        sb.reverse();

        while (sb.length() > 1 && sb.charAt(0) == '0') {
            sb.deleteCharAt(0);
        }

        return sb.toString();
    }
}